1. Field of the Invention
The present invention relates to a method for manufacturing semiconductor substrate, a method for manufacturing semiconductor device, a semiconductor substrate, and a semiconductor device.
2. Description of the Related Art
A semiconductor substrate using a nitride semiconductor is used in a power device or the like which operates with a high frequency and a high output power. In particular, as a power device which is suitable to perform amplification in a high-frequency band of microwaves, submillimeter waves, millimeter waves, and others, for example, a high electron mobility transistor (HEMT) or the like is known.
As the semiconductor substrate using the nitride semiconductor, there is known a semiconductor substrate having an initial layer, a GaN layer, and a barrier layer composed of AlGaN sequentially stacked on a Si substrate.
In a lower layer (a high-resistance layer) in the GaN layer, when electrical resistances in a longitudinal direction and a lateral direction are increased, off characteristics of a transistor can be improved, and a longitudinal direction leak current is suppressed, thereby realizing a high breakdown voltage. Thus, carbon is doped, a deep level is formed in a GaN crystal, and n-type conduction is suppressed.
On the other hand, an upper layer (approximately 1 μm) in the GaN layer functions as a channel layer and it can be a cause of current collapse (a phenomenon that reproducibility of output current characteristics is degraded) when a level to trap a carrier is formed, and hence concentration of carbon and the like is sufficiently reduced (see Patent Literature 1-3).